Lead azide detonators



Patented Mar. 15, 949

LEAD Azrnn DETONATORS Leon Rubenstein and Bertram Campbell, Saltcoats. Scotland. assignors to Imperial Chemical Industries Limited, a corporation of Great Britain No Drawing.

Application November 13, 1946,

Serial No. 709,440. In Great Britain November 3 Claims.

The present invention is concerned with improvements in' or relating to lead azide detonator primary initiating compositions and to the production of lead azide detonators containing the improved compositions in compressed form as a primary initiating charge.

The object of the invention is to improve the cohesive properties of said compositions when compressed into detonators, whereby the hazard associated with the handling of lead azide detonators. more especially during the manufacture of lead electric detonators, may be minimised.

Owing to the poor binding properties of lead azide under compression it is a frequent practice to include a small proportion of tetryl in lead azide detonator primary initiating compositions but it would be desirable to efiect a further improvement over that obtained with tetryl, in the binding properties of primary initiating compositions comprising lead azide.

We have now found that satisfactory detonator primary initiating compositions comprising lead azide having improved cohesive properties when compressed into a detonator are obtained by the inclusion in the compositions of a samll proportion or what may be termed ultra-short fibre nitrocellulose.

The present invention thus comprises detonator primary initiating compositions comprising lead azide and including a small proportion of an ultra-short" fibre nitrocellulose distributed in admixture through the composition. The invention also includes the application of said compositions in the production of detonators and the resulting detonators containing the compositions in compressed form as their primary initiating charges.

By "ultra-short fibre" nitrocellulose as defined herein we mean nitrocellulose fibre having a length between 30 mu and 250 mu, and preferably between 50 mu and 150 mu. Ultra-short fibre" nitrocellulose is preferably prepared by subjectin: nitrocellulose to.viscosity reduction treatments suificiently intense to bring its viscosity as measured in a concentration of 40 gm. in 100 .Iccio'f aqueous acetone made by mixing 95 cc.

acetone with 5 cc. water (aqueous acetone of specific gravity 0.8097 at 20) between the limits 0.3 and 0.5 c. g. s. units at 20 C. The "ultra-short fibre nitrocellulose so prepared may advantageously contain a stabiliser, such as calcium carbonate incorporated with it to the extent of about 0.5 to 1%, since it must conform to existing requirements for stability.

The amount of "ultra-short fibre nitrocellulose may advantageously be from about 4 to 12% by weight of the primary initiating composition. Ultra-short fibre nitrocellulose may advantageously be mixed with the lead azide and any other ingredients of the composition in the dry way.

The ultra-short fibre nitrocellulose may for instance be prepared by directly kiering a fibrous nitrocellulose under pressure until the required fibre length is attained, and then washing the product, but other methods of viscosity reduction than pressure kiering also available, for instance a treatment with boiling dilute aqueous ammonia at ordinary pressure with subsequent bleaching treatment at raised temperature and stabilisation by water boiling. Thus a 10% slurry in a dilute (0.3%) ammonia solution may be made from an industrial nitrocellulose of 12% nitrogen content and a viscosity is measured in a solution containing 40 grams nitrocellulose in cc. of aqueous acetone as aforesaid of about 10 c. g. s. units, the ammoniacal nitrocellulose slurry being boiled at atmospheric pressure for about 16 hours nitrocellulose being then drained, washed and subjected to successive boiling treatments of about 2 hours duration in hypochlorite solution containing 2% available chlorine with an interposed washing treatment, whereby the viscosity is reduced to about 0.4 c. g. s. units. The nitrocellulose is again washed and stabilised by giving it about 4 successive water boilings, each of 4 hours duration. The yield amounts to about half the original weight of the nitrocellulose. In loading the detonator the lead azide-nitrccellulose composition is introduced on to the top of a secondary initiating charge of, for example, tetryl, compressed in the copper or aluminum detonator and compressed into the detonator at the usual pressure employed for loading lead azide primary initiating detonator charges.

The invention is further illustrated by the following examples in which the parts are parts by weight:

Example L Lead azide 64.8 parts, aluminum powder 2.8 parts, lead styphnate 27.6 parts, ultra-short fibre nitrocellulose containing a half per cent of its weight of calcium carbonate 4.8 parts. The detonating composition is made by dry mixing. For the manufacture of detonators it may be introduced on top of a secondary initiating charge of tetryl compressed in the copper or aluminum detonators and compressed into the detonators at the usual pressure employed for loading lead azide primary initiating detonator charges.

Example II The procedure and. the ingredients are the same as in Example I, but they are present in the proportions leadazide 80 parts, aluminuml part,.

lead styphnate 'iB parts, "ultra-short 'tlbr'e"nitro'- cellose '7 parts.

Example III The procedure is the same as in ExampleIand II, but composition of the primarydnitiatingcharge is lead azide 90 parts;fultra-shoi't."fibre nitrocellulose 10 parts. In thls case ai'x initiating;

charge of a demagratable material mays'auvantae geously be compressed over the primary initiating charge.

We claim: 7

1. A detonator primary initiating composition comprising a mixture of lead azide and from 41in. 12 percent of fibrous nitrocellulose, thefibre length of the nitrocellulose being between the range-of trauma t'o zso mieronm- 2. The detonator primary initiating composition claimed in claim 1, wherein the fibre length is from 50 to 150 microns.

3. A detonator assembly containing as the primary: initiating? composition? a compressed mix tureof lead aaide and front er-t6 12 parts-byx-weight of fibrous nitrocellulose, the fibre length of said nitrocellulose being within the range of 30 to 250 microns LEON BERNSTEIN. BERTRAM CAMPBELL.

REFERENCES CITED The following references are of record in the 

